Autopilot takes control of the accelerator, brakes and steering, but asks the driver keeps hold of the wheel.

A driver has been killed in a motorway accident while his Tesla Model S drove itself in Autopilot mode, where its computers, cameras and sensors take control of the accelerator, brakes and steering. It is the first known death in over 130 million miles driven worldwide by the Autopilot system.

The collision occurred in Williston, Florida, on 7 May and killed the driver and owner, 40-year-old Joshua Brown, a well-known member of the Tesla community who regularly posted videos of his car and its Autopilot system to YouTube. The US National Highway Traffic Safety Administration (NHTSA) has opened a preliminary investigation into Autpilot's actions in the moments before the crash.

A blog post published by Tesla on 30 June explains what happened. While travelling along the highway with Autopilot in control, Brown's Model S encountered a large truck which was turning across the road ahead.

The white colour of the truck's trailer, combined with a bright sky, meant neither the car nor driver saw the trailer until it was too late.

The Tesla partially passed under the trailer, until the windscreen made contact with it. According to a police report, the Model S then drove under the trailer, left the side of the road, smashed through two fences and struck a power pole. Its final resting place, according to police, was some 100 feet south of the highway, and Brown died at the scene.

Tesla described the incident as having "extremely rare circumstances", as the gap between the trailer's front and rear wheels was wide enough for the car to drive through, meaning the full force of the impact was met by the windscreen and A-pillars, none of the energy-absorbing front crash structure.

As Tesla noted: "Had the Model S impacted the front or rear of the trailer, even at high speed, its advanced crash safety system would likely have prevented serious injury as it has in numerous similar incidents."

The Model S is widely regarded as one of the safest cars on the road. According to Tesla CEO Elon Musk, the car's radar system deliberately ignores objects high up, so that overhead road signs do not cause the car to brake.

Although it isn't known if the driver was paying full attention, Tesla explains in its blog post how switching on Autopilot does not mean the driver can divert their attention away from the road ahead and traffic around them. The system is also still in a 'beta' stage of development and is being improved regularly via software updates.

Currently, Tesla says more than 130 million miles have been driven by its cars with Autopilot switched on and there has been only one death. By comparison, the company claims there is, on average, a death on US roads for every 94 million miles driven, and worldwide that figure falls to 60 million.

When switched on, Autopilot asks the driver to keep their concentration on the road and hands on the wheel. If they do not, then the car will sound an alert. If they continue to keep their hands off the wheel, the car will slow to a stop and switch on the hazzard lights.

The company added: "Autopilot is getting better all the time, but it is not perfect and still requires the driver to remain alert. Nonetheless, when used in conjunction with driver oversight, the data is unequivocal that Autopilot reduces driver workload and results in a statistically significant improvement in safety when compared to purely manual driving."